Sunday, March 25, 2012

Origin of breccia pipes in the Colorado Plateau and Paradox basin

I heard from Chet Wallace, an exploration geologist from Colorado, who is organizing an informal field trip to the areas around the San Rafael Swell in Utah to examine relations among hydrocarbon generation, metal-bearing reduced brine, burial diagenesis, alteration diagenesis, stratabound mineral deposits, and hydrocarbon reservoirs in a basin analysis context.

Chet told me that breccia pipes [right, surface expression of a breccia pipe in Arizona] occur in several places in the Paradox Basin, and in the past most of these structures have been considered collapse features. He regards these features as dilatant initially, and formed where over-pressured oil-field brine and hydrocarbons penetrated seals at places of weakness in the subsiding Paradox Basin (seal-bypass systems of Cartwright and others, 2007, AAPG Bull.). Chet thinks the breccia pipes in northern Arizona are similar to pipes that penetrated the Messinian evaporite seal in the Levant Basin in the eastern Mediterranean Sea, as described by Cartwright and others (2007) using 3-D seismic analyses. His hypothesis for the northern Arizona scenario is that the evaporite seal that was probably at the top of the Mississippian carbonates is now gone, with only hypogene karst remaining.

For two days they will be in the field to examine aspects of basinal brine and hydrocarbon migration on clastic and carbonate rocks in the Paradox Basin. They will go to Temple Mountain on May 10, and on May 11 we will look at hypogene karst and tar sand, as well as compare alteration diagenesis of structural compartments that partly controlled brine and hydrocarbon migration. Similar structural compartments occur elsewhere in the Paradox Basin.

In an email to the attendees, Chet wrote:

At Temple Mountain the sand injectite, which had been called a "collapse structure" by previous authors, formed as a fluidized autobreccia that was injected upward about 490 m from the White Rim Sandstone. Overpressured reduced, warm, metal-bearing brine and hydrocarbons powered the sand injectite that invaded semi-consolidated overlying strata. A sand injectite is a class of Fluid-Expulsion Structure--FES (or Seal-Bypass System in oil and gas parlance). We will examine the breccia and host rocks, as well as host strata of stratabound mineral deposits near Temple Mountain. On the San Rafael Swell, we'll look at two places that show different aspects of hypogene karst in the Black Box "Dolomite" (Kaibab equivalent). In general, hypogene karst forms at burial depths of 1,500 - 3,000 m in subsiding sedimentary basins, and this karst results from oxidation-reduction reactions between gypsum-anhydrite and hydrocarbons. These chemical reactions affected gypsum-anhydrite strata throughout the Paradox Basin, but hypogene karst has gone unrecognized for the most part. In the process of examining hypogene karst, we'll contrast diagenetic styles of two structural compartments.